Digital convergence in a projection television receiver can be achieved using a two dimensional matrix of adjustable factors applicable to points distributed at regular intervals across the visible screen area. The degree of deflection correction may be finely adjusted at each of these points independently.
In the horizontal direction the deflection correction is determined by a numeric digital factor applicable at the matrix points, which is converted to an analog signal for driving a convergence correction coil. At intermediate points between the points on the matrix, the correction factor is determined by averaging with an analog filter. In the vertical direction it is necessary to calculate the values for the intervening scan lines between the lines corresponding to the points on the correction matrix. In a lower cost display system, correction for the lines between the points that are numerically defined are determined by taking a difference between the correction values for the nearest adjustment point above and below the point in question, dividing by the number of lines in between the adjustment points, and weighting the correction value for the particular line being determined by linear interpolation. Thus the waveform that is generated follows a straight line between the two numeric points. To facilitate interlace scanning an offset value may be added to the correction data for alternate fields.
Digital correction may affect picture geometry in addition to converging corresponding points on the three color rasters. Green is typically chosen to be centered on the projection system optical axis. In this position, the image on the face of the green tube suffers least geometric distortion. Red and blue displays are positioned on the axis vertically but typically are located off the optical axis horizontally. As a result, the red and blue rasters are additionally distorted and require keystone shaping to compensate for this off axis projection location. Because the optical distortion is minimum for the green image, it is chosen as the geometric reference. The green raster is sized and shaped by correction waveforms to minimize geometric distortion. The red and blue rasters are then matched to align precisely with the green image.
The uncorrected green raster suffers a large vertical pincushion distortion which is corrected by a correction waveform. For optimum geometry, the correction waveform along each column has a distinct S-shape having sinusoidal and parabolic components, and for the off axis red and blue images an additional linear component is required.
The correction waveform may, for example, be adjustable along a matrix of factors defining 13 rows and 16 columns. For each point in the matrix, numerical factors define the associated displacement of the red, green and blue rasters to be effected at that point for achieving accurate picture geometry and alignment of the red and blue rasters. Since the number of correction points or nodes for each column is relatively small, for example twelve points spaced vertically in the visible area of the screen, the linearly interpolated S-shaped correction waveform may have abrupt changes in slope as it crosses each node. These slope changes at each adjustment line cause blocks of raster scan lines to appear with a brightness difference due to non-uniform spacing of the horizontal lines in adjacent areas where convergence is adjusted according to different matrix factors. If a video signal having a constant level or a “flat” field is displayed, the raster will exhibit a series of distinct horizontal bands or lines of differing brightness resulting from correction waveform discontinuities.